Generally, in image reading apparatuses provided in image forming apparatuses such as copiers, fax machines or digital multi-functional peripherals, or image reading apparatuses connected to a computer via communication means such as a network, light reflected from an original that is illuminated by an illuminating apparatus including a light source is read as the original image.
For example, a conventional image reading apparatus includes an illuminating apparatus in which a light source for illuminating an original placed on a platen glass and a first mirror are arranged, second and third mirrors, an imaging lens and an imaging element (for example, a line sensor such as a CCD (Charge Coupled Device)). Many of such image reading apparatuses are configured to read an original image by causing light reflected from the original illuminated by a light source to travel via a first mirror, a second mirror, a third mirror and then an imaging lens to form an image on an imaging element.
Also, an image reading apparatus is used as an image reading means when information of an image formed on an imaging element such as a CCD is converted to electronic signals and subjected to image processing, and transferred to an image forming portion that prints image information or to a computer (for example, personal computer) connected to a network, for example.
Conventionally, a bar-shaped light source such as a halogen lamp or a xenon lamp, or a light source in which a plurality of light-emitting elements such as light-emitting diodes (LEDs) are arrayed may be employed as a light source provided in illuminating apparatuses.
In the case where a light source in which a plurality of light-emitting elements are arrayed is employed, light-emitting elements such as LEDs have strong directivity characteristics in a predetermined direction, and thus uneven illuminance corresponding to the pitch between the light-emitting elements may occur on the light-irradiated face of the original.
For example, as the pitch between the light-emitting elements increases, uneven illuminance on the light-irradiated face of the original becomes more noticeable. Therefore, it is preferable to decrease the pitch. However, if the pitch is decreased, the number of necessary light-emitting elements increases, which invites an increase in cost.
Also, as the optical axis distance of the light-emitting elements to the original decreases, uneven illuminance becomes more noticeable. Therefore, it is preferable to increase the optical axis distance. However, if the optical axis distance is increased, the illuminance on the light-irradiated face of the original decreases, and thus it is necessary to increase the light amount emitted by the light-emitting elements.
From a viewpoint of mitigating uneven illuminance described above, Patent Document 1 described below discloses an illuminating apparatus in which a diffusion member for diffusing light is provided between an original and a light source in which a plurality of light-emitting elements are arrayed. Also, Patent Document 2 described below discloses a light irradiating apparatus in which a plurality of light-emitting elements are arranged in a zigzag manner.
However, with the illuminating apparatus disclosed in Patent Document 1, although uneven illuminance on the light-irradiated face of the original caused by the light-emitting elements can be mitigated by providing the diffusion member between the original and the light source, it is necessary to newly provide the diffusion member, which increases the manufacturing cost. In addition, the illuminance on the light-irradiated face of the original is reduced by the diffusion member, which causes a loss in the light amount when light from the light source is irradiated onto the original. For this reason, it is difficult to apply the illuminating apparatus to image reading apparatuses whose original reading speed is comparatively fast. Also, there is an issue that if the light amount of the light-emitting elements is increased for covering the loss in the light amount, the power consumption increases.
Also, with the light irradiating apparatus disclosed in Patent Document 2, even if the light-emitting elements are disposed in a zigzag manner, if the pitch between the light-emitting elements and the optical axis distance from the light-emitting elements to the original are not in an optimal relationship in the state in which uneven illuminance on the light-irradiated face of the original caused by the light-emitting elements is mitigated, it is impossible to efficiently irradiate light from the light-emitting elements onto the light-irradiated face of the original, while the uneven illuminance is mitigated.
Accordingly, it is desired to achieve an optimal relationship between the pitch and the optical axis distance such that the light from the light-emitting elements is efficiently irradiated onto the light-irradiated face of the original, in the state in which the uneven illuminance on the light-irradiated face of the original caused by the light-emitting elements is mitigated, without providing the diffusion member between the original and the light source in which a plurality of light-emitting elements are arrayed.
In this respect, Patent Document 3 described below discloses an illuminating apparatus which is capable of irradiating an original with light without uneven illuminance, while suppressing an increase in cost, by setting the light amount of a single light-emitting element when reading an image (indicated by “A”), the total light amount (indicated by “B”), the irradiation angle of the light-emitting element (indicated by “a” (rad)), the pitch between the light-emitting elements (indicated by “P”), and the distance from the emission face of the light-emitting element to the original face (indicated by “H”) so as to satisfy the relational expressions, A/B≧0.5 and P/H≦0.6α+0.25.